Relaxation-corrected ecg-triggering and navigator-gating technique

ABSTRACT

A system and method for recording magnetic resonance images with reduced relaxation-related artifacts. The system and method improve the conventional methods for acquiring magnetic resonance images of in vivo tissue by augmenting the procedures for eliminating artifacts caused by motion with procedures for eliminating artifacts caused by spin of the magnetic resonance-active nuclei in the specimen of interest. One procedure to eliminate such spin inhomogeneities is to require a delay defined by the time N*T 1 , where N is a numerical value greater than or equal to 5 and T 1  is the characteristic time constant for decay of spins back to the equilibrium longitudinal state. Another procedure uses a value of N less than 5.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of co-pending U.S. patent applicationSer. No. 13/080,281, filed Apr. 5, 2011, and claims priority to and thebenefit thereof, which application is incorporated herein by referencein its entirety.

FIELD OF THE INVENTION

The invention relates to NMR imaging systems and methods in general andparticularly to a Multi Two Dimensional (M2D) NMR system and method.

BACKGROUND OF THE INVENTION

When investigators perform in viva MR imaging of thoracic organs,movement of tissues is caused by the cycle of muscular contractions ofthe atria and ventricles of the heart, and by the cycle of respirationcreated by movement of the diaphragm, even while the subject isnominally in a resting condition. These two sources of movement cancause motion artifacts in MR images. Previous work providedmethodologies to reduce or eliminate artifacts in image acquisitioncaused by cardiac and respiratory movements. See for example, Wang Y,Rossman P J, Grimm R C, Riederer S J, Ehman R L. Navigator-echo-basedreal-time respiratory gating and triggering for reduction of respirationeffects in Coronary Artery Disease 437 three-dimensional coronary MRangiography. Radiology 1996; 198:55-60; Danias P G, McConnell M V,Khasgiwala V C, Chuang M L, Edelman R R, Manning W J. Prospectivenavigator correction of image position for coronary MR angiography.Radiology 1997; 203:733-736; McConnell M V, Khasgiwala V C, Savord B Jet al. Prospective adaptive navigator correction for breath-hold MRcoronary angiography. Magn Reson Med 1997; 37:148-152; McConnell M V,Khasgiwala V C, Savord B J, Chen M H, Chuang M L, Edelman R R, Manning WJ. Comparison of respiratory suppression methods and navigator locationsfor MR coronary angiouaphy. AJR Am J Roentgenol 1997; 168:1369-1375; andKotys M S, Herzka D A, Vonken E J, Ohayon J, Heroux J, Gharib A M,Stuber M, Pettigrew R I. Profile order and time-dependent artifacts incontrast-enhanced coronary MR angiography at 3T: origin and preventionMagn Reson Med. 2009 August; 62(2):292-9.

Cardiac-Gating Technique

The effect of cardiac movement can be reduced by using ECG datacollected from the subject to trigger MR image acquisition. By using ECGdata to trigger image acquisition, acquisition can be acquired only whenthe heart is in diastole. This technique is called cardiac triggering,and serves to reduce the cardiac motion artifacts that would otherwisebe observed.

Diaphragm-Gating Technique

Respiratory movement is controlled for with the so-called navigatortechnique. In the navigator technique, the position of the top of thediaphragm is monitored with a navigator “pencil beam” radio-frequency(RF) pulse that measures the location of the dome of the right side ofthe diaphragm. Using this information on diaphragm position, MR imageacquisition is only performed when the diaphragm is in a predeterminedwindow of location. Specifically, in the conventional navigator gatingmethod, a free navigator is performed to get the most stable andconsistent diaphragm position. The acquired data is only accepted whenthe diaphragm is within the navigator window. If there is motion outsideof the navigator window, the scanner software rejects the acquired data.Additional data samples to make up for the rejected data are thenacquired until the data are sampled when the diaphragm is in positionwithin the navigator window.

When the ECG-triggering and navigator-gating techniques are combined,they can control for the movements of the cardiac and respiratorycycles. However, they do not control for another source of artifact,namely, that resulting from variation in spin condition of nuclei in thetissues being imaged.

When an RF signal acquisition step (also referred to as a “shot” or as a“normal shot”) is conducted, it affects the condition of the spin ofnuclei in the tissues being imaged, causing a deviation from the relaxedposition. A “shot” can be a series of RF pulses in the case of a fastimaging sequence. If the spin of nuclei is not folly relaxed when thenext shot is conducted, an artifact is created. For example, if an M2Dand single-shot turbo field echo (TFE) pulse sequence, combined with thenormal ECG-triggering and navigator-gating techniques, is used toacquire sagittal heart images, the intensity of different slices isdifferent, which results in a severe “banding artifact” once thetransverse or coronal images are reconstructed from the sagittal images.This banding artifact impairs the ability of clinicians to accuratediagnose heart disease. In the M2D and single-shot turbo field echo(TFE) pulse sequence method, multiple image slices are excited one afterthe other and each slice is acquired within one heart beat.

Also known in the prior art is Pines et al., U.S. Pat. No. 7,061,237,issued Jun. 13, 2006, which is said to disclose an apparatus and methodfor remote NMR/MRI spectroscopy having an encoding coil with a samplechamber, a supply of signal carriers, preferably hyperpolarized xenonand a detector allowing the spatial and temporal separation of signalpreparation and signal detection steps. This separation allows thephysical conditions and methods of the encoding and detection steps tobe optimized independently. The encoding of the carrier molecules maytake place in a high or a low magnetic field and conventional NMR pulsesequences can be split between encoding and detection steps. In oneembodiment, the detector is a high magnetic field NMR apparatus. Inanother embodiment, the detector is a superconducting quantuminterference device. A further embodiment uses optical detection ofRb—Xe spin exchange. Another embodiment uses an optical magnetometerusing non-linear Faraday rotation. Concentration of the signal carriersin the detector can greatly improve the signal-to-noise ratio.

There is a need for systems and methods for providing improved in vivomagnetic resonance images of subjects.

SUMMARY OF THE INVENTION

According to one aspect, the invention features an improvement in amagnetic resonance imaging system having a computer-based controlapparatus that controls the timing of the taking of magnetic resonanceimages based on a muscular state of a body part of a living subject. Theimprovement comprises a set of instructions recorded on amachine-readable medium, the set of instructions when operatingconfigured to control the operation of the system in response to twoconditions to provide two mutually exclusive outcomes, so as to permitthe taking of a magnetic resonance image (a “shot”) when both a muscularcondition is within an acceptable range and when, for all shots after afirst shot, a time elapsed since the last shot was taken is at leastN*T₁, where T₁ is a time constant representative of a decay rate for aspecimen of interest in a magnetic resonance process and N represents anumerical value greater than one, and N*T₁ is of sufficient duration toallow a plurality of nuclear spins in said specimen of interest to decayback to a statistically defined spin state after the last shot; andleave the spin state of the plurality of nuclear spins undisturbed whenat least one of the muscular conditions is not within the acceptablerange and, for all shots after a first shot, the condition that the timeelapsed since the last shot is at least N*T₁ is not satisfied; and theset of instructions configured to provide as output at least one of animage displayed to a user, a stored image, and transmission of the imageto a remote location for storing or viewing.

In one embodiment, the statistically defined spin state is anequilibrium state.

In another embodiment, the statistically defined spin state is a spinstate that is a statistically similar spin state to a spin state thatexisted at the start of a previous shot.

In yet another embodiment, a zero flip angle shot is applied to leavethe plurality of nuclear spins undisturbed,

In a further embodiment, a shot is omitted to leave the plurality ofnuclear spins undisturbed.

In one embodiment, the set of instructions is configured to control theoperation of the system in an iterative manner.

In another embodiment, the set of instructions is configured to controlthe operation of the system iteratively to take a number of shotssufficient to provide an image of the body part of the living subject.

yet another embodiment, the body part is cardiac tissue.

In yet another embodiment, the body part is diaphragm tissue.

In still another embodiment, the value of N is approximately 5.

In yet other embodiments, the value of N is less than 5, and theplurality of nuclear spins is in a statistically similar spin state to aspin state that existed at the start of a previous shot.

According to another aspect, the invention relates to an improvement ina magnetic resonance imaging method operating on a computer-basedcontrol apparatus that controls the timing of the taking of magneticresonance images based on a muscular state of a body part of a livingsubject. The improvement comprises the steps of determining whether ornot a muscular condition of a body part of a living subject is within anacceptable range; determining whether or not, for all shots after afirst shot, a time elapsed since the last shot was taken is at leastN*T₁, where T₁ is a time constant representative of a decay rate for aspecimen of interest in a magnetic resonance process and N represents anumerical value greater than one, and N*T₁ is of sufficient duration toallow the nuclear spins in the specimen of interest to decay back to astatistically defined spin state after the last shot; if bothdetermining steps result in a positive outcome, permitting the taking ofa magnetic resonance image (a “normal shot”); if either determining stepresults in a negative outcome, inhibiting the taking of a “normal shot”,leaving the spin state of the nuclear spins undisturbed, and repeatingthe two determining steps until both outcomes are positive; and afterthe step of permitting the taking of a magnetic resonance image isperformed, providing as output at least one of an image displayed to auser, a stored, image, and transmission of the image to a remotelocation for storing or viewing.

In one embodiment, leaving the spin state of the plurality of nuclearspins undisturbed is accomplished by applying a zero flip angle shot.

In another embodiment, leaving the spin state of the plurality ofnuclear spins undisturbed is accomplished by omitting a shot,

In one embodiment, the body part is cardiac tissue.

In yet another embodiment, the body part is diaphragm tissue.

In another embodiment, the value of N is approximately 5.

In a different embodiment, the value of N is less than 5, and theplurality of nuclear spins is in a statistically similar spin state to aspin state that existed at the start of a previous shot.

In yet another embodiment, the step of permitting the taking of amagnetic resonance image performed in an iterative manner.

In still another embodiment, the method further comprises repeating thetwo determining steps and the permitting step until an entire image ofinterest is acquired.

The foregoing and other objects, aspects, features, and advantages ofthe invention will become more apparent from the following descriptionand from the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The objects and features of the invention can be better understood withreference to the drawings described below, and the claims. The drawingsare not necessarily to scale, emphasis instead generally being placedupon illustrating the principles of the invention. In the drawings, likenumerals are used to indicate like parts throughout the various views.

FIG. 1A is a graph showing the variation in signal intensity versusdistance across a phantom as recorded using the normal for conventional)ECG-triggering and navigator-gating method for recording MR images.

FIG. 1B is a graph showing the variation in signal intensity versusdistance across the phantom as recorded using the Relaxation-CorrectedECG-Triggering and Navigator-Gating Technique according to the presentdescription for recording MR images.

FIG. 1C is a reconstructed MR image of a phantom from the sagittalimages acquired by the normal ECG-triggering and navigator-gatingmethod.

FIG. 1D is a corresponding image using the Relaxation-CorrectedECG-Triggering and Navigator-Gating Technique.

FIG. 2A is an in vivo coronal magnetic resonance image reconstructedfrom the sagittal images recorded using conventional ECG-Triggering andNavigator-Gating MRI techniques.

FIG. 2B is an in vivo coronal magnetic resonance image reconstructedfrom the sagittal images recorded using the Relaxation-Corrected.ECG-Triggering and Navigator-Gating Technique.

FIG. 2C is an in vivo transverse magnetic resonance image reconstructedfrom the sagittal images recorded using conventional ECG-Triggering andNavigator-Gating MRI techniques.

FIG. 2D is an in vivo transverse magnetic resonance image reconstructedfrom the sagittal images recorded using the Relaxation-CorrectedECG-Triggering and Navigator-Gating Technique.

FIG. 3 is a graph showing the evolution of the magnetization M(t) withtime after a 90 degree shot, in which a T₁ value of 1471 ms was used tosimulate the relaxation of heart at 3T scanner.

FIG. 4 is a diagram that illustrates the system-control scheme (orsystem-control logic) and also represents a flow diagram of the processby which the improvement in making magnetic resonance images isimplemented.

DETAILED DESCRIPTION

We have developed a technique, called the Relaxation-CorrectedECG-Triggering and Navigator-Gating Technique, to eliminate the artifact(for example, the banding artifact in FIGS. 1C, 2A and 2C) caused byvariation in the condition of the spin of hydrogen in tissues that arebeing observed. This Relaxation-Corrected ECG-Triggering andNavigator-Gating Technique is believed to be novel because it controlsfor variation in the condition of spin relaxation in addition toadjusting for the motion of the cardiac and respiratory cycles.

By correcting for variation in spin relaxation occurring over the courseof image acquisition, the Relaxation-Corrected ECG-Triggering andNavigator-Gating Technique eliminates a source of image artifacts, whichis an additional source of bias that was not eliminated by earliertechniques, and thus the technique improves the quality of images. Thisimproved quality can lead to improved diagnosis and therapy assessmentfor patients with a wide range of medical conditions.

As used herein, the acronym MR refers to magnetic resonance, the acronymMRI refers to magnetic resonance imaging, and the acronym NMR refers tonuclear magnetic resonance.

The new methodology for two-dimensional imaging significantly improvesupon current ECG-triggering and navigator-gating techniques byimplementing additional gating of image acquisition via synchronizationof the relaxation condition of the spin of nuclei with the cardiac andrespiratory cycles. This relaxation-corrected technique accomplishesimproved signal acquisition by further restricting the image slices thatare used for final MR images.

The Relaxation-Corrected ECG-Triggering and Navigator-Gating Techniqueis implemented through a software adjustment on conventional MRIscanners. Validation experiments performed at UMass Medical School, onphantoms and in vivo, have demonstrated that the technique is highlyeffective. The improved signal that results from this technique isillustrated in FIG. 1B, FIG. 1D, FIG. 2B, and FIG. 2D.

FIG. 1A is a graph showing the variation in signal intensity versusdistance across a phantom as recorded using the normal (or conventional)ECG-triggering and navigator-gating method for recording MR images.

FIG. 1B is a graph showing the variation in signal intensity versusdistance across the phantom as recorded using the Relaxation-CorrectedECG-Triggering and Navigator-Gating Technique according to the presentdescription for recording MR images.

FIG. 1C is an MR image of a phantom reconstructed from the sagittalimages acquired by the normal ECG-triggering and navigator-gatingmethod. FIG. 1D is a corresponding image using the Relaxation-CorrectedECG-Triggering and Navigator-Gating Technique. As is clearly seem theimage in FIG. 1D is significantly improved with regard to bandingartifacts as compared to the image in FIG. 1C, i.e., the bandingartifacts are eliminated by the Relaxation-Corrected ECG-Triggering andNavigator-Gating Technique.

The images were reconstructed from the sagittal images acquired by a M2D(Multi-2 Dimension) and single-shot TEE pulse sequence, combined withthe normal ECG-triggering and navigator-gating techniques or theRelaxation-Corrected ECG-Triggering and Navigator-Gating Technique. Forphantom experiments, the trigging signal was provided by thephysiological simulation of the scanner and the diaphragm motion wassimulated by manually moving a water phantom.

FIG. 2A, FIG. 2B, FIG. 2C, and FIG. 2D compare in vivo MR images of ahuman subject using the normal ECG-triggering and navigator-gatingmethod, with images using the Relaxation-Corrected ECG-Triggering andNavigator-Gating Technique. The images in FIG. 2B and FIG. 2D producedwith use of the Relaxation-Corrected ECG-Triggering and Navigator-GatingTechnique are of far higher quality, with significantly reduced bandingartifacts as compared to the corresponding FIG. 2A and FIG. 2C,respectively, which were recorded using conventional ECG-triggering andnavigator-gating method on the same instrument.

The relaxation-corrected ECG-triggering and navigator-gating techniquewas validated, for M2D-Turbo Field Echo (TEE) imaging of the heart. Inthe M2D imaging method, multiple image slices are excited one after theother. It is expected that the technique can also be adapted for othertypes of movement in other organs throughout the thoracic and abdominalcavities. It is expected that the technique can be applied for use withother MR imaging methods, including 3-D imaging, spiral imaging, radialimaging, multi-slice interleaved imaging, multi-shot imaging, spin-echoimaging, and coronary artery imaging.

In the Relaxation-Corrected ECG-Triggering and Navigator-GatingTechnique, when the navigator pulse assessing diaphragm position occursright before a shot (which is a condition termed a “leading navigator”),if the diaphragm is found to be out of the navigator window, thetechnique uses a “dummy shot” with a Flip Angle (EA) of 0° instead of a“real shot” or “normal shot.” Alternatively, one can just skip a shot.This innovation keeps the spin of nuclei in the tissue being imaged atthe equilibrium state until the diaphragm is found in the navigatorwindow and a real shot is conducted. Without this correction, a realshot would change the state of the spin, creating a confound forsubsequent shots. With this improvement, all the spins are kept atequilibrium before real signal acquisition is performed.

Mathematical Description

A brief explanation of NMR using hydrogen, as an example is nowpresented, as described in an MRI physics course (in notes by JerryAllison, Chris Wright, and Tom Lavin, of the Department of Radiology ofthe Medical College of Georgia), NMR using hydrogen is well known tothose of ordinary skill in the nuclear magnetic resonance arts. Atomsthat contain odd numbers of protons or neutrons (or both) possessnuclear magnetic moments, whereas atoms that have even numbers ofprotons and neutrons have no spin and are not observed in NMR methods.Hydrogen (having one proton and no neutron), and its isotopes deuterium(one proton and one neutron) and tritium (one proton and two neutrons)all can be observed using NMR. Hydrogen is much more abundant thaneither deuterium or tritium, so those isotopes will not be addressedhere in detail. Hydrogen is present in water (H₂O) and various organiccompounds present in the body. One can expect that the hydrogen willprovide significant NMR signals. In fact, hydrogen is used as areference material for NMR signal, generally providing a signal that ismuch larger than essentially every other NMR-active nucleus. However,¹⁹F, ³He, ³¹ P, ¹²⁹Xe, ⁷Li, ¹⁷O, and other nuclei also give readilyobservable NMR signals and can be used for imaging and spectroscopy.

Hydrogen nuclei have only two available spin states. Hydrogen is said tohave nuclear spin of ½. The two possible spin states are spin up (lowenergy state: parallel to applied static magnetic field) and spin down(high energy state: antiparallel to applied static magnetic field). Theenergy difference between the spin up and spin down states, denoted byΔE=hν, is directly proportional to the applied magnetic field, whereh=Planck's constant (6.62×10⁻³⁴ J s), and ν=spin frequency (cycles/s, orHertz). When RE energy is supplied at the resonant frequency of thenucleus, known as the Larmor frequency, the nuclei can change statebetween the up and down conditions. A transition from spin up to spindown absorbs energy (as the nucleus increases in energy), which atransition from spin down to spin up releases energy.

The Larmor equation, given below, describes the resonant precessionalfrequency of a nuclear magnetic moment in an applied static magneticfield.

ω=γB_(o)

where ω=precessional frequency (resonant frequency), γ=gyromagneticratio (MHz/Tesla), and B_(o)=magnetic field (Tesla). The Larmorfrequency of hydrogen in a field of 1.5 Tesla is 63.87 MHz, while theLarmor frequency of deuterium in a 1.5 Tesla field is 9.795 MHz. Thisresonant frequency difference makes distinguishing hydrogen fromdeuterium a simple matter.

Conventional RF coils and RF electronics used in MRI are tuned for anarrow band of RF frequencies. Hydrogen is commonly imaged in MRIbecause of its high sensitivity and abundance. To convert from imaging Hto imaging another nuclear substance, one would need to employ RF coilsand RF electronics that can be tuned for the alternate frequency.

According to electromagnetic theory, RF excitation can be described as arotating magnetic field (and electric field) in the plane perpendicularto the static magnetic field B_(o). RF excitation is produced byapplying an oscillating voltage waveform to an RF exciter (transmitter)coil. The magnetic field component that rotates in the transverse planeduring RF excitation is referred to as the B₁ magnetic field.

Frequently, the macroscopic magnetization is spiraled down until itprecesses in the transverse plane (plane perpendicular to the staticmagnetic field). This is called a 90° flip. After a 90° flip, themacroscopic magnetization is precessing entirely in the transverse planeat the Larmor frequency and there are equal numbers of nuclei in thespin up and spin down states.

After the 90° flip, the longitudinal component of the magnetization inthe direction of the static magnetic field (B_(o)) is zero. Themacroscopic magnetization prior to a 90° flip is entirely longitudinaland is said to point along the “Z” axis. Following a 90° flip,magnetization is entirely transverse and is said to rotate or precess inthe transverse plane defined by the “X” and “Y” axes. From thetransverse magnetization state, the spin population relaxes to thethermal equilibrium magnetization condition, in which an equilibrium ofup and down spins exists, as given by the well-known Boltzmann relation.A free-induction decay (FID) signal arises representing the relaxationof the transverse magnetization is induced in the RF coil.

The spin component in the direction of the static magnetic field B_(o)returns exponentially to thermal equilibrium magnetization with rateconstant T₁. T₁ is a measure of the time required to re-establishthermal equilibrium between the spins and their surroundings. T₁increases as the magnetic field increases.

The mechanism behind this phenomenon can be expressed as follows. Aftera 90 degree RF shot is applied, the longitudinal magnetization willrelax to the equilibrium state as a function of time expressed by theequation

M(t)=Mo(1−exp(−t/T ₁))

where Mo is the magnetization during the equilibrium state, t is time,and T₁ is a time constant representative of a decay rate for a specimenin a magnetic resonance process. If t is less than about 5 times largerthan T₁, then M(t) will not be fully relaxed. In this case, the next RFshot will result in a signal that is less than the first one. In thecase of M2D heart imaging, the TR value, which is the time intervalbetween the two shots and is decided by the heart rate, is about 1second, which is smaller than the T₁ value of the heart tissue, which isabout 1471 ms on a 3T scanner.

FIG. 3 is a graph showing the evolution of the magnetization M(t) withtime. For the purposes of illustrating the evolution of M(t), we havetaken the value of Mo as being nominally 100 units. The time spacing twas taken as 200 increments of 50 milliseconds (ms) each. For hearttissue, with T₁=1471 ms, the units of relative time (t/T₁) along thex-axis range from 0/1471=0 to 10000/1471=6.7981. The x-axis labels rangefrom 0.00 to 6.66 but several additional time unit marks are shown. Ascan be seen (or as can be deduced by performing the calculation ofM(t)), M(t) attains the values shown in Table I.

TABLE I t/T₁ (dimensionless units) M(t) as percentage of Mo 1 63.212 286.466 3 95.021 4 98.168 5 99.326 6 99.752 7 99.909

FIG. 4 is a diagram that illustrates the system-control scheme (orsystem-control logic) and also represents a flow diagram of the processby which the improvement in making magnetic resonance images isimplemented. In FIG. 4 at step 410, the muscular condition of a subjectis assessed to determine if a suitable “shot” or image element can betaken, or a series of shots, in the case of a fast imaging sequence. Asdescribed hereinabove with regard to cardiac imaging, the condition ofthe cardiac muscles and the diaphragm muscles are taken intoconsideration. For making magnetic resonance images of some other partof the body, such as some other organ in the subject, other muscularcondition might need to be assessed.

At step 420, a decision is taken as to whether both the necessarymuscular conditions exist (e.g., that the muscular condition of the bodypart(s) of interest are within an acceptable range or “window”) and, forall shots after a first shot, that a time long enough has elapsed sincethe last shot. The time that is considered to be long enough is given byN*T₁, where N represents a numerical value greater than one and ofsufficient size to allow the nuclear spins of interest to decay back toan equilibrium state after a shot. The numerical value N is clearly alower bound, as waiting longer between shots (e.g., after theequilibrium is effectively attained) is also acceptable in principle. Asindicated hereinabove, for cardiac tissue, N is approximately 5, but itshould be understood that N does not need to be an integer. In someembodiments, N can also be less than 5, but N*T₁ should be equal orsimilar for all shots so that the spin states of nuclei in the objectbeing measured are in a statistically similar condition for each shot,which one can consider to be a defined spin state. In the defined spinstate, a plurality of nuclear spins is in a statistically equivalentspin state (or statistically similar spin state) to a spin state thatexisted at the start of a previous shot.

If the outcome of the decision at step 420 is that either the muscularcondition is not within the window or that the time elapse is not yet atleast N*T₁, the flow proceeds according to the arrow marked NO thatconnects step 420 to step 430. At step 430, the nuclear spins are leftundisturbed. This can be accomplished by applying a zero flip angleshot. Alternatively, this can be accomplished by skipping a shot. Theprocess then follows arrow 435 back to step 410, where the condition ofthe subject is again assessed.

If the outcome of the decision at step 420 is that both the muscularcondition is within the range or window and that the time elapsed sincethe last “shot” is at least N*T₁, the flow proceeds according to thearrow marked YES that connects step 420 to step 440.

At step 440 the system performs a “shot” to make another magneticresonance image (or image section) of the body portion of interest ofthe subject. Process flow then takes the process to step 450.

At step 450 a decision is taken as to ‘Whether the last image “shot”that is needed to prepare an image of the body part of the subject hasbeen taken. If the result of the decision at step 450 is that the last“shot” has been taken (e.g., that sufficient images of suitable qualityhave been taken to allow a complete image of interest to be prepared),the image data is processed as needed at step 460, and an image (and asdesired, the data representative of the image) are treated to provide atleast one of an image displayed to a user, a stored image (and as may bedesired or needed, stored data), and transmission of the image (and asdesired or required, the image data) to a remote location for storing orviewing.

If the decision at step 450 is that the last “shot” needed to make theimage of interest has not yet been taken, the system follows arrow 435back to step 410, where the condition of the subject is again assessed,and additional image data can be taken.

Our technique is useful for the M2D signal-shot TFE method. In variousembodiments of our method, the slice order could be of any kind. Apreferred embodiment uses an interleaved order. It is believed that aninterleaved order is advantageous to overcome the leakage of excitation.In other embodiments, K-space encoding can be any kind of order as well.

Trailing Navigator

When a navigator pulse assessing diaphragm position occurs right after ashot (a condition termed a “trailing navigator”), a different methodwould have to be used because the real shot was performed before thenavigator was performed. In such a case, if the trailing navigatorindicates the diaphragm is out of the navigator window, the data wouldnot be used and the scanner would proceed to sampling the next slice,thus avoiding bias from the change in spin caused by the real shot thatcame prior to the trailing navigator. If the image data corresponding toa specific slice were omitted, data would then be re-acquired for thatoriginal slice after all other shots for all other slices wereperformed, because the spins would have had a chance to return toequilibrium by that time. But for the last five shots, they should betreated as the leading navigator method to save time. In someembodiments, it is expected that the leading and trailing navigatorsampling will be combined, providing the most robust data in terms ofcontrolling for both diaphragm position and spin relaxation. In thiscase, the method for leading navigator and trailing navigator should becombined,

Definitions

Unless otherwise explicitly recited herein, any reference to anelectronic signal or an electromagnetic signal (or their equivalents) isto be understood as referring to a non-volatile electronic signal or anon-volatile electromagnetic signal,

Recording the results from an operation or data acquisition, such as forexample, recording results at a particular frequency or wavelength, isunderstood to mean and is defined herein as writing output data inanon-transitory manner to a storage element, to a machine-readablestorage medium, or to a storage device. Non-transitory machine-readablestorage media that can be used in the invention include electronic,magnetic and/or optical storage media, such as magnetic floppy disks andhard disks; a DVD drive, a CD drive that in some embodiments can employDVD disks, any of CD-ROM disks (i.e., read-only optical storage disks),CD-R disks (i.e., write-once, read-many optical storage disks), andCD-RW disks (i.e., rewriteable optical storage disks); and electronicstorage media, such as RAM, ROM, EPROM, Compact Flash cards, PCMCIAcards, or alternatively SD or SDIO memory; and the electronic components(e.g., floppy disk drive, DVD drive, CD/CD-R/CD-RW drive, or CompactFlash/PCMCIA/SD adapter) that accommodate and read from and/or write tothe storage media. Unless otherwise explicitly recited, any referenceherein to “record” or “recording” is understood to refer to anon-transitory record or a non-transitory recording,

As is known to those of skill in the machine-readable storage mediaarts, new media and formats for data storage are continually beingdevised, and any convenient, commercially available storage medium andcorresponding read/write device that may become available in the futureis likely to be appropriate for use, especially if it provides any of agreater storage capacity, a higher access speed, a smaller size, and alower cost per bit of stored information. Well known oldermachine-readable media are also available for use under certainconditions, such as punched paper tape or cards, magnetic recording ontape or wire, optical or magnetic reading of printed characters (e.g.,OCR and magnetically encoded symbols) and machine-readable symbols suchas one and two dimensional bar codes. Recording image data for later use(e.g., writing an image to memory or to digital memory) can be performedto enable the use of the recorded information as output, as data fordisplay to a user, or as data to be made available for later use. Suchdigital memory elements or chips can be standalone memory devices, orcan be incorporated within a device of interest, “Writing output data”or “writing an image to memory” is defined herein as including writingtransformed data to registers within a microcomputer.

“Microcomputer” is defined herein as synonymous with microprocessor,microcontroller, and digital signal processor (“DSP”). It is understoodthat memory used by the microcomputer, including for exampleinstructions for data processing coded as “firmware” can reside inmemory physically inside of a microcomputer chip or in memory externalto the microcomputer or in a combination of internal and externalmemory. Similarly, analog signals can be digitized by a stand-aloneanalog to digital converter (“ADC”) or one or more ADCs or multiplexedADC channels can reside within a microcomputer package, it is alsounderstood that field-programmable array (“FPGA”) chips orapplication-specific integrated circuits (“ASIC”) chips can performmicrocomputer functions, either in hardware logic, software emulation ofa microcomputer, or by a combination of the two. Apparatuses having anyof the inventive features described herein can operate entirely on onemicrocomputer or can include more than one microcomputer.

General-purpose programmable computers useful for controllinginstrumentation, recording signals, and analyzing signals or dataaccording to the present description can be any of a personal computer(PC), a microprocessor-based computer, a portable computer, or othertype of processing device. The general-purpose programmable computertypically comprises a central processing unit, a storage or memory unitthat can record and read information and programs using machine-readablestorage media, a communication terminal such as a wired communicationdevice or a wireless communication device, an output device such as adisplay terminal, and an input device such as a keyboard. The displayterminal can be a touch screen display, in which case it can function asboth a display device and an input device. Different and/or additionalinput devices can be present such as a pointing device, such as a mouseor a joystick, and different or additional output devices can be presentsuch as an enunciator, for example a speaker, a second display, or aprinter. The computer can run any one of a variety of operating systems,such as for example, any one of several versions of Windows, or ofMacOS, or of UNIX, or of Linux, or of Solaris. Computational resultsobtained in the operation of the general-purpose computer can be storedfor later use, and/or can be displayed to a user. At the very least,each microprocessor-based general purpose computer has registers thatstore the results of each computational step within the microprocessor,which results are then commonly stored in cache memory for later use.

Many functions of electrical and electronic apparatuses can beimplemented in hardware (for example, hard-wired logic), in software(for example, logic encoded in a program operating on a general purposeprocessor), and in firmware (for example, logic encoded in anon-volatile memory that is invoked for operation on a processor asrequired). The present invention contemplates the substitution of oneimplementation of hardware, firmware, and software for anotherimplementation of the equivalent functionality using a different one ofhardware, firmware, and software. To the extent that an implementationcan be represented mathematically by a transfer function, that is, aspecified response is generated at an output terminal for a specificexcitation applied to an input terminal of a “black box” exhibiting thetransfer function, any implementation of the transfer function,including any combination of hardware, firmware, and softwareimplementations of portions or segments of the transfer function, iscontemplated herein, so long as at least some of the implementation isperformed in hardware.

Theoretical Discussion

Although the theoretical description given herein is thought to becorrect, the operation of the devices described and claimed herein doesnot depend upon the accuracy or validity of the theoretical description.That is, later theoretical developments that may explain the observedresults on a basis different front the theory presented herein will notdetract from the inventions described herein.

Any patent, patent application, or publication identified in thespecification is hereby incorporated by reference herein in itsentirety. Any material, or portion thereof, that is said to beincorporated by reference herein, but which conflicts with existingdefinitions, statements, or other disclosure material explicitly setforth herein is only incorporated to the extent that no conflict arisesbetween that incorporated material and the present disclosure material.In the event of a conflict, the conflict is to be resolved in favor ofthe present disclosure as the preferred disclosure.

While the present invention has been particularly shown and describedwith reference to the preferred mode as illustrated in the drawing, itwill be understood by one skilled in the art that various changes indetail may be affected therein without departing from the spirit andscope of the invention as defined by the claims.

1. In a magnetic resonance imaging system having a computer-basedcontrol apparatus that controls the timing of the taking of magneticresonance images based on a muscular state of a body part of a livingsubject, the improvement comprising: a set of instructions recorded on amachine-readable medium, the set of instructions when operatingconfigured to control the operation of the system in response to twoconditions to provide two mutually exclusive outcomes, so as to:determine whether or not a muscular condition of a body part of a livingsubject is within an acceptable range; permit the taking of a magneticresonance image (a “shot”) when both said muscular condition is withinsaid acceptable range and when, for all shots after a first shot, anelapsed time since the last shot was taken is at least N*T₁, where T₁ isa time constant representative of a decay rate for a specimen ofinterest in a magnetic resonance process and N represents a numericalvalue greater than one, and N*T₁ is of sufficient duration to allow aplurality of nuclear spins in said specimen of interest to decay back toa statistically defined spin state after the last shot; and omit a shotto leave said spin state of said plurality of nuclear spins undisturbedwhen at least one of said muscular conditions is not within saidacceptable range and, for all shots after a first shot, said conditionthat said time elapsed since said last shot is at least N*T₁ is notsatisfied; and said set of instructions configured to provide as outputat least one of an image displayed to a user, a stored image, andtransmission of an image to a remote location for storing or viewing. 2.The magnetic resonance imaging system of claim 1, wherein saidstatistically defined spin state is an equilibrium state.
 3. Themagnetic resonance imaging system of claim 1, wherein said statisticallydefined spin state is a spin state that is a statistically similar spinstate to a spin state that existed at the start of a previous shot. 4.(canceled)
 5. (canceled)
 6. The magnetic resonance imaging system ofclaim 1, wherein said set of instructions is configured to control theoperation of the system in an iterative manner.
 7. The magneticresonance imaging system of claim 1, wherein said set of instructions isconfigured to control the system to take a number of shots sufficient toprovide an image of said body part of the living subject.
 8. Themagnetic resonance imaging system of claim 1, wherein said body part iscardiac tissue, a muscular state of said cardiac tissue being determinedusing an ECG-triggering technique and an acceptable range for saidECG-triggering being a heart in diastole.
 9. The magnetic resonanceimaging system of claim 1, wherein said body part is diaphragm tissue, amuscular state of said diaphragm tissue being determined using anavigator-gating technique and an acceptable range for saidnavigator-gating technique being said diaphragm tissue within anavigator window.
 10. The magnetic resonance imaging system of claim 1,wherein said value of N is approximately
 5. 11. The magnetic resonanceimaging system of claim 1, wherein said value of N is less than 5, andsaid plurality of nuclear spins is in a statistically similar spin stateto a spin state that existed at the start of a previous shot.
 12. In amagnetic resonance imaging method operating on a computer-based controlapparatus that controls the timing of the taking of magnetic resonanceimages based on a muscular state of a body part of a living subject, theimprovement comprising the steps of: determining whether or not amuscular condition of a body part of a living subject is within anacceptable range; determining whether or not, for all shots after afirst shot, a time elapsed since the last shot was taken is at leastN*T₁, where T₁ is a time constant representative of a decay rate for aspecimen of interest in a magnetic resonance process and N represents anumerical value greater than one, and N*T₁ is of sufficient duration toallow a plurality of nuclear spins in said specimen of interest to decayback to a statistically defined spin state after the last shot; if bothdetermining steps result in a positive outcome, permitting the taking ofa magnetic resonance image (a “normal shot”); if either determining stepresults in a negative outcome, inhibiting the taking of a “normal shot”,leaving said spin state of said nuclear spins undisturbed by omitting ashot, and repeating the two determining steps until both outcomes arepositive; and after said step of permitting the taking of a magneticresonance image is performed, providing as output at least one of animage displayed to a user, a stored image, and transmission of an imageto a remote location for storing or viewing.
 13. (canceled) 14.(canceled)
 15. The magnetic resonance imaging method of claim 12,wherein said body part is cardiac tissue, a muscular state of saidcardiac tissue being determined using an ECG-triggering technique and anacceptable range for said ECG-triggering being a heart in diastole. 16.The magnetic resonance imaging method of claim 12, wherein said bodypart is diaphragm tissue, a muscular state of said diaphragm tissuebeing determined using a navigator-gating technique and an acceptablerange for said navigator-gating technique being said diaphragm tissuewithin a navigator window.
 17. The magnetic resonance imaging method ofclaim 12, wherein said value of N is approximately
 5. 18. The magneticresonance imaging method of claim 12, wherein said value of N is lessthan 5, and said plurality of nuclear spins is in a statisticallysimilar spin state to a spin state that existed at the start of aprevious shot.
 19. The magnetic resonance imaging method of claim 12,wherein said step of permitting the taking of a magnetic resonance imageperformed in an iterative manner.
 20. The magnetic resonance imagingmethod of claim 12, further comprising repeating the two determiningsteps and the permitting step until an entire image of interest isacquired.